The present invention relates generally to providing network services such as load balancing, packet filtering or Network Address Translation (NAT). More specifically, network services are provided using service managers that send instructions to forwarding agents that are integrated into a routing infrastructure. The forwarding agents dispatch packets using Tag switching according to the instructions received from service managers.
As the IP protocol has continued to be in widespread use, a plethora of network service appliances have evolved for the purpose of providing certain network services not included in the protocol and therefore not provided by standard IP routers. Such services include NAT, statistics gathering, load balancing, proxying, intrusion detection, and numerous other security services. In general, such service appliances must be inserted in a network at a physical location where the appliance will intercept all flows of interest for the purpose of making its service available.
FIG. 1 is a block diagram illustrating a prior art system for providing a network service. A group of clients 101, 102, and 103 are connected by a network 110 to a group of servers 121, 122, 123, and 124. A network service appliance 130 is physically located in the path between the clients and the servers. Network service appliance 130 provides a service by filtering packets, sending packets to specific destinations, or, in some cases, modifying the contents of packets. An example of such modification would be modifying the packet header by changing the source or destination IP address and the source or destination port number.
Network service appliance 130 provides a network service such as load balancing, caching, or security services. In providing security services, network service appliance 130 may function as a proxy, a firewall, or an intrusion detection device. For purposes of this specification, a network service appliance that acts as a load balancer will be described in detail. It should be noted that the architecture and methods described are equally applicable to a network service appliance that is functioning as one of the other above described devices.
Network service appliance 130 is physically located between the group of servers and the clients that they serve. There are several disadvantages to this arrangement. First, it is difficult to add additional network service appliances when the first network service appliance becomes overloaded because the physical connections of the network must be rerouted. Likewise, it is difficult to replace the network service appliance with a back up network service appliance when it fails. Since all packets pass through the network service appliance on the way to the servers, the failure of the network service appliance may prevent any packets from reaching the servers and any packets from being sent by the servers. Such a single point of failure is undesirable. Furthermore, as networks and internetworks have become increasingly complex, multiple services may be required for a single network and inserting a large number of network service appliances into a network in places where they can intercept all relevant packet flows may be impractical.
The servers may also be referred to as hosts and the group of servers may also be referred to as a cluster of hosts. If the group of servers has a common IP address, that IP address may be referred to as a virtual IP address (VIPA) or a cluster address. Also, it should be noted that the terms client and server are used herein in a general sense to refer to devices that generally request information or services (clients) and devices that generally provide services or information (servers). In each example given it should be noted that the roles of client and server may be reversed if desired for a particular application.
A system that addresses the scalability issues that are faced by network service appliances (load balancers, firewalls, etc.) is needed. It would be useful to distribute functions that are traditionally performed by a single network element and so that as much function as possible can be performed by multiple network elements. A method of coordinating work between the distributed functions with a minimum of overhead is needed.
Although network service appliances have facilitated the development of scalable server architectures, the problem of scaling network service appliances themselves and distributing their functionality across multiple platforms has been largely ignored. Network service appliances traditionally have been implemented on a single platform that must be physically located at a specific point in the network for its service to be provided.
For example, clustering of servers has been practiced in this manner. Clustering has achieved scalability for servers. Traditional multiprocessor systems have relatively low scalability limits due to contention for shared memory and I/O. Clustered machines, on the other hand, can scale farther in that the workload for any particular user is bound to a particular machine and far less sharing is needed. Clustering has also facilitated non-disruptive growth. When workloads grow beyond the capacity of a single machine, the traditional approach is to replace it with a larger machine or, if possible, add additional processors within the machine. In either case, this requires downtime for the entire machine. With clustering, machines can be added to the cluster without disrupting work that is executing on the other machines. When the new machine comes online, new work can start to migrate to that machine, thus reducing the load on the pre-existing machines.
Clustering has also provided load balancing among servers. Spreading users across multiple independent systems can result in wasted capacity on some systems while others are overloaded. By employing load balancing within a cluster of systems the users are spread to available systems based on the load on each system. Clustering also has been used to enable systems to be continuously available. Individual application instances or machines can fail (or be taken down for maintenance) without shutting down service to end-users. Users on the failed system reconnect and should not be aware that they are using an alternate image. Users on the other systems are completely unaffected except for the additional load caused by services provided to some portion of the users that were formerly on the failed system.
In order to take full advantage of these features, the network access must likewise be scalable and highly available. Network service appliances (load-balancing appliances being one such example) must be able to function without introducing their own scaling limitations that would restrict the throughput of the cluster. A new method of providing network services using a distributed architecture is needed to achieve this.
When network services such as load balancing are provided by such a distributed system, a method of directing a packet to a different network device than the network device specified by the destination IP address contained in the packet IP header is needed. For example, a packet header may contain the IP address of a virtual machine and it may be necessary to direct the packet to a specific real machine. A flexible method of directing packets to different network devices is needed in order to efficiently provide network services. Specifically, a method is needed for forwarding packets to destinations other than the one specified in the packet without requiring the use of special purpose software at the destination.
A flexible method of directing packets to different network devices is described. A service manager provides instructions to forwarding agents implemented on routers, switches, or other network devices. The instructions indicate how packets for certain flows are to be handled. Packets may either be directed or dispatched by a forwarding agent. Directed packets are forwarded to the network device that corresponds to the destination IP address in the packet. The service manager may specify that the destination IP address is to be changed. If the dispatched packets are not subnet adjacent, then the dispatched packets are forwarded using Tag Switching to a network device specified by the service manager. Packets are thus efficiently forwarded in a manner that is transparent to destination servers and that does not restrict network design. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device, a method, or a computer readable medium such as a computer readable storage medium or a computer network wherein program instructions are sent over optical or electronic communication links. Several inventive embodiments of the present invention are described below.
In one embodiment, a method of providing a network service includes storing at a forwarding agent a tag table. The tag table includes a list of tags and each tag specifies an interface and a destination. An instruction is received from a service manager specifying an overriding packet destination. The overriding packet destination is a destination that corresponds to a specific destination in a specific one of the tags. A packet is dispatched from the forwarding agent to the overriding packet destination using the specific one of the tags in response to the instruction from the service manager.
In another embodiment, a forwarding agent that redirects packets to an overriding destination according to a packet dispatching instruction received from a service manager includes a memory including a forwarding agent tag table wherein the tag table includes a list of tags and wherein each tag specifies an interface and a destination. An instruction interface is configured to receive packet dispatching instructions from a service manager specifying an overriding packet destination wherein the overriding packet destination is a destination that corresponds to a specific destination in a specific one of the tags. A memory is configured to store the packet dispatching instruction and a processor is configured to match the packet dispatching instructions to a packet and to dispatch the packet from the forwarding agent to the overriding destination using the specific one of the tags in response to the packet dispatching instruction.
In another embodiment, a system for providing load balancing among a group of servers connected to a network includes a forwarding agent implemented on a router that is part of the network. The forwarding agent includes a memory that includes a forwarding agent tag table. The tag table includes a list of tags and each tag specifies an interface and a destination that corresponds to one of the servers. The forwarding agent is further configured to dispatch packets according to a dispatch instruction. A service manager is configured to select a specific server from the group of servers and to send a packet dispatching instruction that specifies the specific server as a packet dispatch destination. The service manager is further configured to send the dispatch instruction to the forwarding agent that links the specific server to a packet flow. Thus, packets in the packet flow are forwarded by the forwarding agent in accordance with the instruction.